One of my goals was to make a system that I could easily remove upon moving day and leave the next homeowner with a standard "dumb" thermostat.

That objective forces me to switch 24VAC to turn on the fan, furnace and compressor. A Digikey propaganda email pointed me to some Panasonic solid state relays on a SMT package. Those clocked in around $7 each, further searching yielded:

http://www.digikey.com/product-detail/en/AQY282SX/255-3393-2-ND/646353

For less than a $1 a output I can fully isolate the 24VAC from my board. Who said promo emails never yielded anything useful?